The underlying hypothesis of this research program is that sleep, shallow torpor, and hibernation are homologous physiological phenomena and are all intimately related to thermoregulatory functions. The major objectives will be (a) to investigate the thermoregulatory adjustments and metabolic changes during the electrophysiological states of sleep in humans exposed to various ambient temperatures, and (b) to determine whether shallow torpor represents an extension of thermoregulatory adjustments normally occurring during sleep states. Specific studies will be conducted to determine: (1) Changes in metabolism, vasomotor tone, and skin and rectal temperatures during stages of sleep at a thermoneutral temperature of 29 degrees. (2) How stages of sleep, thermoregulatory mechanisms and metabolism are influenced by ambient temperatures above and below thermoneutrality (37 degrees, 34 degrees, and 21 degrees, 24 degrees, respectively). (3) Whether adaptation occurs to chronic exposure to cold at night in thermoregulatory and sleep-stage variables. (4) Whether pocket mice and hummingbirds enter shallow torpor through sleep, and if so, whether thermoregulatory adjustments during entrance correlate with changes in sleep states. We believe that this research should yield new insights into the interrelations between sleep, torpor, hibernation and thermoregulation and about the evolution and biological functions of sleep.